1. Field of the Invention
The present invention relates to an optical pickup device by which at least one of reproduction and recording of information can be performed in an optical disc as an optical information recording medium, and an objective lens unit used for the optical pickup device.
2. Description of Related Art
Conventionally, optical discs have been known as an information recording medium. For example, Compact Disc (hereinafter also referred to as CD) and Digital Versatile Disc (hereinafter also referred to as DVD) have been widely used. Furthermore, a high density optical disc (hereinafter also referred to as next-generation DVD) using an objective lens having a numeric aperture (NA) of 0.8 or more also has been commercially available. These optical discs are based on different standards and thus have different distances from the surface of a disc to an information recording surface, different wavelengths of laser light to be used, and different NAs required for an objective lens for example. Thus, an optical pickup device that can access the plurality types of optical discs also have been marketed.
An objective lens included in an optical pickup device is designed so as to be movable in a direction perpendicular to a recording surface, for the purpose of focusing to a recording surface of an optical disc. An objective lens included in an optical pickup device is also designed so as to be movable in a radial direction for the purpose of tracking the optical disc.
With regards to the optical pickup device as described above, an optical pickup device as disclosed in Japanese Patent Unexamined Publication No. 2004-311004 for example has been known. According to this optical pickup device, a standard of an inserted optical disc is determined by detecting reflected light from the optical disc while allowing the objective lens to have a minimum number of numeric apertures to determine the type of the optical disc based on the detection result, thereby suppressing the objective lens from colliding with the disc surface.
Another optical pickup device as disclosed by Japanese Patent Unexamined Publication No. 2005-93070 for example also has been known. According to this optical pickup device, an optical head section supported by a wire suspension is selectively positioned and fixed while being spaced from an optical disc with a predetermined distance so that collision between the optical head section and a surface of the optical disc due to external impact can be prevented.
FIG. 12 is a view showing a schematic cross section of an optical disc D in the thickness direction. Although FIG. 12 exemplarily illustrates a CD, the dimension and scale in FIG. 12 are different from the actual ones.
In FIG. 12, the optical disc D has a disk-like shape having a diameter of 120 mm and includes a base made of polycarbonate resin or the like by an injection molding. Reference Numeral 1 denotes a surface of a base 2 from which information is read. Reference Numeral 3 denotes a reflective film of an aluminum film formed on an information recording surface. Reference Numeral 4 denotes a protection film provided on the reflective film 3. Reference Numeral 5 denotes a printing surface. Reference Numeral 6 denotes a stack ring. Reference mark H denotes a center hole. A region extending from a center C in a concentric manner and having a radius from 13 mm to 16.5 mm is a clamp region.
As shown in FIG. 12, the optical disc D includes a stack ring (also called as stack rim) 6 in order to prevent an information reading surface from being damaged when the optical disc D is stacked together with a plurality of optical discs. The stack ring 6 is formed to have a convex shape that has a radius from a center C of the optical disc D of 16.5 mm to 22 mm, that is concentric with the optical disc D, and that protrudes from a surface 1 from which information is read. An area external to a circle having a radius of 23 mm from the center C of the optical disc D is determined as a region to which information is recorded.
When the optical disc D having the shape as descried above is recorded with more information in such a manner that an objective lens is positioned closely as much as possible to an information recording region ranging from the center C to a radius of 23 mm, a problem that a risk in which the objective lens unit including a lens frame may interfere with the stack ring 6 protruding from the information reading surface 1 is increased, is caused.
This problem is more severe because when an objective lens has a short focal distance, a working distance (WD) (i.e., a distance between a surface of the optical disc and a final surface of the objective lens or a distance between the surface of the optical disc and a flange surface of the objective lens) is short.
Furthermore, when a versatile objective lens that can be used with a plurality types of optical discs is used with a disc based on a standard in which a thickness from an information-reading surface to an information recording surface is thick, an objective lens unit is closer to the information-reading surface of the disc. As a result, a higher risk of an interference between the objective lens and the stack ring, is caused.
The optical pickup devices described in the above patent documents are for the collision with the information-reading surface and thus cannot solve the problem of the interference between the stack ring and the objective lens unit.